1. Field of the Invention
The invention relates to the production of nitrogen. More particularly, it relates to a membrane utilizing process and system for the production of low cost, high purity nitrogen.
2. Description of the Prior Art
The production of high purity nitrogen has, for many years, been carried out employing state-of-the-art air separation technology based on cryogenic distillation techniques. Because of the favorable economics of scale-up for such cryogenic distillation, large tonnage nitrogen users are supplied with nitrogen gas piped from a cryogenic plant installed on the users' site. Smaller tonnage users, i.e., 2-30 tons/day or less, are typically supplied with liquid nitrogen trucked to the users' site from a centrally located liquid nitrogen production plant. The cost of liquefying nitrogen gas and of transporting the liquid nitrogen from an off-site cryogenic plant to the users' site will be seen to add significantly to the cost of the nitrogen as supplied to the user.
In recent years, therefore, a major challenge in the art has been to develop small tonnage air separation plants that can effectively produce low cost nitrogen gas at the users' site. Recent developments relating to pressure swing adsorption (PSA) and membrane technologies have served to significantly lower the cost of on-site systems for the production of low purity, small tonnage nitrogen. On the other hand, high purity nitrogen cannot be economically produced by such PSA or membrane systems because of practical limitations rendering the power requirements and the cost of such systems prohibitive.
There is a desire in the art for the development of membrane or PSA systems and approaches capable of reducing the cost of on-site, high purity nitrogen. One approach that has been employed to reduce the cost of said on-site, high purity nitrogen involves the use of a membrane or PSA system coupled with a trace oxygen removal system for final purification of the nitrogen product. In this approach, a membrane or PSA system is used for initial air separation to produce nitrogen with about 1,000 ppm up to about 50,000 ppm of oxygen. A catalyst system, e.g. a Deoxo system, is then used to remove additional oxygen to produce a purified nitrogen product stream having a residual oxygen content of 10 ppm or less. While this approach enables high purity nitrogen to be produced on-site at a lower cost than by membrane or PSA systems alone, the cost saving achieved thereby nevertheless represents only a marginal improvement over that associated with the supply of liquid nitrogen by truck to the users' site. This is primarily due to the relatively high cost of the hydrogen required to react with the oxygen present in the partially purified nitrogen stream for a removal thereof. This approach could be of more practical commercial significance as compared to the trucking of liquid nitrogen, however, if a low cost hydrogen supply source and more efficient means of utilizing hydrogen were available at the users' site.
There are presently a number of industrial applications, particularly in the petrochemical industry, which require high purity nitrogen and that also have low cost hydrogen available on site. Frequently, however, this low cost, available hydrogen is impure and contains various hydrocarbons. As the use of such impure hydrogen would be disadvantageous with respect to the operation of a catalyst system for final nitrogen purification, such impure hydrogen is purified, and the resulting high purity hydrogen is used in an efficient and effective manner in said catalyst system for final, on-site nitrogen purification.
The inherent simplicity of permeable membrane systems provides a strong incentive and desire in the art for the development of such systems and related processes for the on-site production of high purity nitrogen. Those skilled in the art will also appreciate that there are particular overall processing operations for which an on-site PSA system is more appropriate than a membrane system, despite the inherent simplicity of membrane systems. The desire in the art for improved overall membrane and PSA systems for on-site, high purity nitrogen production will thus be seen as involving, in approaches utilizing a catalyst system for final nitrogen production, the development of means to effectively utilize hydrogen in the Production of nitrogen in a commercially feasible and efficient manner.
In cryogenic air system operations, a Deoxo system can be used to catalytically remove oxygen from an inert gas (argon) stream. When hydrogen is added to a crude argon stream containing any quantity of oxygen, the hydrogen and oxygen will combine to produce water as said mixture passes over a palladium catalyst. Typical systems operate at relatively constant flows and oxygen concentrations. Hydrogen control is not critical since excess hydrogen is easily removed after the crude argon stream is refined in the Deoxo system and subsequently recondensed. Such a system is typically operated with 1% (10,000 ppm) to 2% (20,000 ppm) "excess" hydrogen concentration above the stoichiometric ratio required for complete oxygen removal.
In the operation of a non-cryogenic (i.e. membrane or PSA) Deoxo system for the separation of nitrogen from air and the purification thereof, the high purity nitrogen produced generally contains less than 5 ppm oxygen, typically less than 1 ppm. While it is desirable that minimal quantities of excess hydrogen be present in said high purity product, no cost effective means have been available to essentially totally remove residual hydrogen from the final high purity nitrogen product, as in the cryogenic processing operations referred to above. There is a need in the art, therefore, to develop means so that the desired quantity of hydrogen for use in the Deoxo system reaction can be accurately introduced into said Deoxo system so that minimal excess hydrogen will be present in the high purity nitrogen product.
It is an object of the invention, therefore, to provide an improved system and process for the production of on-site, high purity nitrogen.
It is another object of the invention to provide an improved overall system and process, utilizing membrane or PSA systems for such on-site, high purity nitrogen production.
It is a further object of the invention to provide a process and system for the non-cryogenic separation of air and the production of high purity nitrogen product containing minimal quantities of excess hydrogen.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.